Japan is gripped by mass allergies. A 1950s project is to blame

A decision made 70 years ago to reforest vast swathes of Japan with just two kinds of tree has come back to haunt the country.

In February, videos showing what looked like waves of smoke blowing off an evergreen forest went viral in Japan. It wasn't smoke – it was pollen, and the videos were a warning to tens of millions of residents of the archipelago nation: prepare your masks and allergy medicine.

Every spring (which is already arriving earlier in Japan due to climate change) you'll see people of all ages wearing masks on the streets of cities across the country. The reason: hay fever, driven by all the pollen. 

Hay fever – also known as allergic rhinitis – has now become a national crisis in Japan, with an estimated 43% of the population experiencing medium to severe symptoms. This compares to 26% in the UK and 12-18% in the US.

As well as the discomfort, these allergies can lead to sleep loss and poor concentration, and sufferers are more likely to experience other conditions such as asthma and food allergies. At the peak of Japan's hay fever season, the economic impact from both sick days and lower consumer spending is estimated at $1.6bn (£1.2bn) per day. 

So why does Japan have such bad allergies? The reason has little to do with poor health or pollution, or even the natural environment, but decisions made by leaders more than 70 years ago in the decades after World War Two.

During the war, oil and gas shortages led Japan to turn to the nation's most abundant natural resource – forests – as a source of fuel for home and industry. The result was widespread deforestation of natural forests, with the mountains around major cities like Tokyo, Osaka and Kobe completely stripped bare of trees.

"After World War Two, many of Japan's mountains became barren, causing disasters in various regions," says Noriko Sato, a professor and forestry researcher at Kyushu University in Fukuoka, Japan. (Bare mountains can increase the incidence of landslides and flooding). "Large-scale afforestation was carried out by public works, funded by tax revenues, to prevent soil erosion." 

Aiming for rapid reforestation, the government chose to plant reams of only two different native, fast-growing evergreen species that could quickly reforest landscapes and provide wood for future use in construction: the Japanese cedar, sugi, and the Japanese cypress, hinoki.

Today, these hinoki and sugi plantation forests still cover around 10 million hectares (25 million acres) – a fifth of Japan's entire land area. 

The problem is, sugi and hinoki trees also produce large amounts of lightweight pollen which can easily drift into cities. It's this pollen, often released all at once from the monoculture plantations, that is responsible for most seasonal allergies in Japan. The issue has become all the worse since these trees release ever more pollen after maturing at 30 years of age – now the case for nearly all of them.

"Pollen allergies have become a national health issue in Japan," says Sato. "Addressing this problem is urgent."

In 2023, Japan declared allergies a national social problem and the central government set out an ambitious plan – reduce pollen by 50% in 30 years. As a first step, it aims to reduce the forest areas planted with sugi trees by 20%.

But swapping out forests covering over 2% of Japan in 10 years is a massive endeavour. Plus, simply cutting these trees down won't be enough – they also need to be replaced with new forests to avoid soil erosion or accidentally undercutting Japan's own climate targets.

Walking through sugi or hinoki plantation forests is eerie – all the trees are the same height and there are few birds or insects. The ground is spongy with dry needles, and there's little light or sound.

It's a stark contrast to Japan's natural forests, which teem with biodiversity and sound. With their diverse tree species like red pine, larch and maple, these forests support more of all kinds of wildlife. Japan's unique geography has made it one of the world's biodiversity hotspots, but habitat loss and invasive species have led much of its unique wildlife to become increasingly at risk.

With the monoculture plantation forests causing so many problems, it makes sense that Japan is now........

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